AMD has been pushing open source and claiming they'll improve their linux drivers for years now. On one hand they have lived up to these promises as usability is much better than it once was.
On the other hand the drivers are still a lot worse than the windows versions and a lot less stable and for gaming nvidia came out from behind and managed to create a closed source driver that works.
So in short a lot of people are getting a little skeptical. In theory AMD is the superior choice due to freedom. In practice if you want to play games with a brand new gpu then you need an nvidia.